A fail safe electric valve actuator mounted to transfer power to a valve stem.
Fail safe valve actuators heretofore devised have generally included a torque limiting device connected directly to the drive shaft of an electric motor for protecting the electric motor from the high torque created when the valve stem abruptly stops moving when the valve member reaches its operating position. These devices generally include a centrifugal brake between the torque limited device and the valve stem to absorb a portion of the energy released when the power fails and the valve stem is moved rapidly to its fail safe position by a spring. The relative positions of the torque limiting device and the centrifugal brake under certain operating conditions did not provide consistent closing speed and did not adequately protect drive systems.
A fail safe valve actuator powered by an electric motor is disclosed in U.S. Pat. No. 5,195,721 entitled xe2x80x9cFAIL SAFE VALVE ACTUATOR.xe2x80x9d The output shaft of the motor is connected to a centrifugal clutch and a torque-limiting device, both located in a centrifugal clutch housing. The output shaft of the torque-limiting device is connected to a pinion in the gear housing portion of the actuator housing.
When centrifugal force is sufficient to provide enough friction to transmit torque without undue slippage between shoes and the clutch housing, the housing rotates with the output shaft of the motor. The torque limiting device is in the upper portion of the centrifugal clutch housing. When the centrifugal clutch is engaged and the motor is running, the clutch housing rotates with a disc as long as detent buttons are in engagement with indentations. As the torque increases, the lateral force between the detent buttons and sloping walls of the indentations increases. When the valve stem stops moving, the detents will be forced laterally out of the indentations to disconnect the electric motor from the output shaft of the clutch.
In a second embodiment of the apparatus discloses in U.S. Pat. No. 5,195,721 a wrap spring clutch is mounted on the ball screw mechanism on the valve stem for holding the valve stem in position and to resist rotation of the valve stem by force applied by the spring in the valve actuator. A centrifugal brake is provided in this embodiment to dissipate the energy and thus slow down the upward movement of the valve stem. The centrifugal brake is in slow speed high torque position, while the wrap spring clutch is mounted in a high torque position.
The output shaft of the torque-limiting device, connected to the input pinion in the gear housing portion of the actuator housing, is in a position where power is transmitted at high speed and relatively low torque, making it difficult to adjust the clutch to provide precision speed control. The relative positions of the torque limiting device and the centrifugal brake under certain operating conditions resulted in excessive wear and maintenance of the clutch and drive system.
The electric fail safe valve actuator disclosed herein relates to improvements in the valve actuator disclosed in U.S. Pat. No. 5,195,721, the disclosure of which is incorporated herein by reference in its entirety for all purposes. Specifically, reference is made to this patent for a description of the details of the construction of the valve actuator connected between the gear train and the valve which is being actuated.
An electrically actuated clutch is located between an electric motor and a gear train such that the torque required from the electrically actuated clutch is reduced. This increases the clutch life and reduces power consumption. The clutch is capable of holding the valve in its operating position during extreme vibration and has a very consistent closing speed. Further, the clutch is configured to hold the valve member in its operating position when the motor is not energized.
A torque-limiting device is connected to the output end of a gear train in a position where power is transmitted at lower speed and at higher torque than in devices heretofore devised. The relative positions of the torque limiting device and the electrically operated clutch enhance the ability to provide consistent closing speed and reduce power consumption. The torque limiting device positioned at or near the output end of the gear train provides added protection for parts of the valve actuator. Further, operating at high torque causes the slip clutch to be less sensitive than torque limiting devices positioned between the electric motor and the input shaft of the gear train which operated at high speed and low torque.
Disclosed herein is an improved method of actuating a valve wherein power is transmitted from a motor through an electrically actuated clutch mechanism that includes an electrically actuated wrap spring clutch and a centrifugal brake to the input end of a drive train, wherein the electrically actuated clutch mechanism is preset to engage for transmitting power to the drive train. The centrifugal brake is actuated only after reaching a predetermined speed to provide smooth engagement for protecting the drive train against shock and high stresses and dissipates energy.
Power is transmitted adjacent the output end of the gear train through a torque limiter, such as a friction overload slip clutch to a ball nut, wherein the torque limiter is preset to limit torque transmitted from the gear train to the ball nut and from the ball nut to the output end of the gear train to prevent damage due to excessive torque generated by overloads.